The role of surfactants in phosphate conversion coatings

Abstract

Surface active agents occupy a predominant role in the metal finishing industry. Being capable of reducing the surface tension and improving the wettability of the surface, the importance of such additives is very well realized and phosphating baths have been developed with surfactants as one of the additives. However, surfactant additives could influence the nucleation and growth of phosphate crystals and alter the kinetics of the phosphating process. Addition of surfactants in phosphating baths is beneficial in obtaining the desired orientation of the phosphate crystal and the crystallites of the base metal, leading to an improved adherence and excellent corrosion resistance of the resultant phosphate coating. The influence of surfactant additives on the nucleation, growth, refinement of crystals and the continuous dissolution and re-precipitation of the phosphate coating, calls for a through understanding of role of surfactant additives often incorporated in phosphating baths so that the desirable qualities can be successfully built-in in the resultant phosphate coating.